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This chapter includes:

  • Effects of the hypothalamus on eating behaviour
  • Recognising facial expressions of emotion
  • The classic case of Phineas Gage
  • The James-Lange theory of emotion
  • Check your understanding and prepare for your exams using the multiple choice, short answer and essay practice tests also available.
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What you should be able to do after reading chapte

  • Define motivation
  • Describe and understand the processes involved in starting and stopping a meal
  • Outline the basic psychology and physiology of thirst
  • Describe the major eating disorders, anorexia nervosa, bulimia nervosa and obesity, and outline their possible causes.
  • Describe and understand the process of sexual development and orientation.
  • Evaluate theories explaining aggression and describe the factors which lead to aggressive behaviour. 
  • Describe the ways in which psychologists have defined and studied emotion.
  • Evaluate the 'fundamental emotion' debate
  • Outline the major theories of emotion
  • Describe current understanding of the biological basis of emotion
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Why do people behave differently? Why do some individuals eat particular foods whereas others eat different foods? Why do we eat in the first place? What makes us attracted to different sexual partners, or any sexual partner? Why do we become aggressive?

Most of these questions can probably be answered by motivation. When commonly used, motivation refers to a driving force that moves us to a particular action. More formally, motivation is a general term for a group of phenomena that affect the nature of an individual's behaviour, the strength of the behaviour and the persistence of the behaviour.

Motivation includes two types of phenomenon. First, stimuli that were previously associated with pleasant or unpleasant events motivate approach or avoidance behaviours. For example, if something reminds you of an interesting person you met recently, you may try to meet that person again by consulting your mobile phone and sending a message. Secondly, being deprived of a particular reinforcer increases an organism's preference for a particular behaviour. Besides obvious reinforcers such as food or water, this category includes more subtle ones. For example, after spending a lot of time performing routine tasks, we become motivated to go for a walk or meet with friends.

Motivation affects all categories of behaviour. 

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Physiology of reinforcement

To understand the nature of reinforcement we must understand something about its physiological basis. Olds and Milner (1954) discovered quite by accident that electrical stimulation of parts of the brain can reinforce an animal's behaviour. For example, rats will repeatedly press a lever when the brain is electrically stimulated.

The neural circuits stimulated by this electricity are also responsible for the motivating effects of natural reinforcers such as food, water or sexual contact, and of drugs such as heroin, alcohol and cocaine. Almost all investigators believe that the electrical stimulation of the brain is reinforcing because it activates the same system that is activated by natural reinforcers and by drugs that people commonly abuse. The normal function of this system is to strengthen the connections between the neurons that detect the discriminative stimulus (such as the sight of a lever) and the neurons that produce the operant response (such as a lever press). The electrical brain stimulation activates this system directly. 

Researchers have discovered that an essential component of the reinforcement system consists of neurons that release dopamine as their transmitter substance. Thus, all reinforcing stimuli appear to trigger the release of dopamine in the brain. 

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The role of conditioned reinforcement

Another phenomenon that affects the tendency to persevere is conditioned reinforcement. When stimuli are associated with reinforcers, they eventually acquire reinforcing properties of their own. For example, the sound of the food dispenser reinforces the behaviour of a rat being trained to press a lever.

Motivation is not merely a matter of wanting to do well and work hard. It also involves the ability to be reinforced by the immediate products of the work being done. If a person has regularly been exposed to particular stimuli in association with reinforcers, that person's behaviour can be reinforced by those stimuli. In addition, if the person has learned how to recognise self-produced stimuli as conditioned reinforcers, the performance of the behaviours that produce them will be 'self-reinforcing'.

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A popular theory at the turn of the twentieth century argued that thirst was caused by a dry mouth and that it was this dryness that regulated how much water we ingested. When the salivary glands reduced the amount of fluid they secreted, this made the mouth dry and was the cue for drinking. While plausible, the theory was not supported by evidence because even if water was made available to the mouth but was prevented from reaching the stomach, drinking would continue: the mouth was being kept wet, not dry and yet drinking continued because the fluid did not reach the stomach. Why?

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What stops a meal?

Nutrient detectors sense the fact that the body's suplies of stored energy are getting low by measuring glucose and fatty acids in the blood. Through their connection with the brain these detectors are able to stimulate hunger. But what ends hunger? What brings a meal to its finish?

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